The Quick Look feature in Mac OS X Lion is useful for previewing images, text files and websites. But you can also use it to quickly check the free space remaining on your hard drive.
Simply click on the Mac HD icon on the Desktop or in a Finder window and hit the spacebar.
Another quick way to check your free space is to turn on the Status Bar in Finder windows by visiting View>Show Status Bar (Command + /) and click on the Mac HD icon in the Finder window. The Status Bar at the bottom of the Finder window will display your free space.